637. 二叉树的层平均值

给定一个非空二叉树, 返回一个由每层节点平均值组成的数组.

示例 1:

输入:
    3
   / \
  9  20
    /  \
   15   7
输出: [3, 14.5, 11]
解释:
第0层的平均值是 3,  第1层是 14.5, 第2层是 11. 因此返回 [3, 14.5, 11].

注意：

节点值的范围在32位有符号整数范围内。

思路：和102二叉树的层次遍历类似，每层遍历结束后存储节点的平均值即可。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def averageOfLevels(self, root):
        """
        :type root: TreeNode
        :rtype: List[float]
        """
        
        if root == None:
            return []
        
        result, temp, queue = [], [], []
        
        current_layer_last, next_layer_last = root, root
        
        queue.append(root)
        
        while queue:
            curr_node = queue.pop(0)
            temp.append(curr_node.val)
            
            curr_node_left = curr_node.left
            curr_node_right = curr_node.right
            
            if curr_node_left:
                queue.append(curr_node_left)
                next_layer_last = curr_node_left
                
            if curr_node_right:
                queue.append(curr_node_right)
                next_layer_last = curr_node_right
                
            if curr_node == current_layer_last:
                current_layer_last = next_layer_last
                result.append(sum(temp)/len(temp))
                temp = []
        
        return result